The PopTrayU installation location, usually a folder called PopTrayU under Program Files, or Program Files (x86) contains a sub-folder called Languages. This folder contains language files for each supported language. Or, files for individual languages can also be downloaded here:
https://sourceforge.net/p/poptrayu/code/HEAD/tree/trunk/installer/Languages/
The file extension is .ptlang which means "PopTrayU language file", but they are just text-files. In older versions of PopTrayU, they are encoded in the code-page of the applicable region, however, the 5.0+ version of PopTrayU use UTF-8 to encode non-english languages, so they can be used by users of any region.
language.ptlang contains a list of the untranslated strings sorted by what version they were added. Locate the version the translation was last updated for, and copy and paste all the strings for one version newer (somewhere in the middle) to the latest (at the end of the file) into your language file. Translate these additional strings, then email (as an attachment) to to jojobear99@users.sourceforge.net
Make a copy of blank.ptlang with the English name of the language as the filename. On the right hand side of each equals sign, type the translation of the phrase. blank.ptlang contains all of the phrases used in the program, (mostly) sorted alphabetically.
Email the completed translation (as an attachment) to to jojobear99@users.sourceforge.net
Although the translation can be done with a text-editor, additional software exists that may make the translation effort easier:
F13 PopTray Language Editor - This editor provides a nice interface for translating the software, with the english and other language side by side in a table.
PopTray Language Checker - This tool will compare the translation file to the current blank translation and identify how many/which phrases are missing from the translation